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News analysis To survive the demands of the Tour de France elite cyclists must do more than train, they must also harbor a set of inborn traits, say experts.

The Tour de France is a gruelling, leg-burning, heart-straining event that challenges cyclists to superhuman feats.

This year's race covers more than 3500 kilometres in three weeks, with only two rest days. By the end of the tour, riders will have climbed 23 mountain passes, some up to 2000 metres high.

The stamina required to do such a thing sounds impossible to most people - and for most of us, it is.

"There is no other sport in the world that continues for 21 days, day in and day out, over mountains, in the rain and heat, over rough terrain," says Hunter Allen, an elite cycling coach and co-author of Training and Racing with a Power Meter. "These guys are the crème of the crème of the crème of the top."

"The guys that win?" he says. "They are genetic mutants. Truly mutants. There aren't many of them on the planet."

Working together

To succeed as a Tour de France rider, the lungs, heart and muscles have to work exceptionally well - both on their own and in coordination with each other, says Suzanne Atkinson, head cycling and triathlon coach at Steel City Endurance in Pittsburgh.

Lungs bring in oxygen. The heart pumps oxygen-filled blood to the muscles. And the muscles use the oxygen to convert fuel to energy. The body of a tour rider does all three better than most.

Lung capacity is probably not much different between someone like Lance Armstrong and an amateur cyclist who rides regularly, Atkinson says. But the amount of blood that comes out of a tour rider's heart with each beat skyrockets during a tough stage, reaching levels far higher than what an average athlete's heart could do.

A 2007 study found that, at their fittest, professional cyclists have hearts that are up to 40% larger than normal.

A tour rider's all-important leg-pumping muscles are also able to use up a larger proportion of the oxygen that reaches them - a measurement called VO2 max. Anyone can improve his or her VO2 max with training, but levels can go higher in some people, including tour riders.

On top of that, elite cyclists can pedal at their highest level of exertion for extremely long periods of time - long past the time when an amateur rider would have to take a break from the burn.

Genetics and training

Getting their bodies to the pinnacle of performance requires hours of training a day, year after year.

"Genetics alone will not create a tour rider," says Atkinson. "A cyclist with the right genetics won't get there without the right training, either."

A range of heights, weights and body shapes show up on the tour, with larger thighs on the best sprinters, for example. But the top overall cyclists want to be as lean as possible to maximise their strength-to-weight ratio and to minimise drag.

At the same time, they don't want to drop too much weight or they'll sacrifice power and strength.

Even if his body is prepared, an elite cyclist will only win the tour if his brain is fit for the task, too.

"I would not ignore the mental side of things, both in terms of willingness to suffer and being tactically smart on which moves to cover and when to attack," says Sam Callan, education manager at USA Cycling in Colorado Springs. "There are a lot of folks with the physiology who do not have the mental acumen."